ooo thats a good one, prepare for a flood of ATI V's Nvidia posts, Most of the people I know who have made a HTPC have used the fanless cards, which have big copper fins on to disperse the heat and then a quiet 120mm fan at the back. have a google around, but I have owned both ATI and Nvidia and I would say they both have their pros and cons, up to you really.
Do these ones with a heatsink take up less room than one with a fan (when vertical)? I only have two PCI and one PCI Express. I use the PCI slots so there is a certain amount of room for a graphics card.
Does it make much of a difference having 512MB RAM over 256MB if I am not into games that much? Will the extra help doing HD resolutions?